Answer:
Due to time required for warmup of land and water.
Explanation:
The hottest day of the year in the northern hemisphere don't fall on the summer solstice i.e. June 21, because the ground and water in the Northern Hemisphere needs time to warm up. It takes 3 weeks or one month to warm up the land and water, once it warm up the hottest day of the year started. so the hottest day of the year in the northern hemisphere comes in the month of July.
